1. Field of the Invention
This invention relates to a non-woven inorganic fiber mat such as a glass fiber mat and to a method and apparatus for the production thereof. The present invention also relates to the use of the mat in building boards, such as gypsum building boards.
2. Description of the Background Art
A particularly useful form of building board is known as glass reinforced gypsum board (GRG). GRG board and its manufacture is described in GB-A-2 053 779. GRG board is of generally conventional appearance and is composed of a gypsum with a non-woven glass mat immediately below one or both principal surfaces. The mat is introduced into the core by vibrating the core slurry, over- or underlain by the mat, to cause it to pass through the mat, so that the surface layer or layers of gypsum are integral with the core. GRG boards are stronger than conventional boards and exhibit superior fire resistance.
In the manufacture of GRG board, the need to provide strength by employing non-woven glass fiber mat of relatively low diameter (for example, 13 μm) fibers conflicts with the need to ensure efficient exhaustion through the mat of air from the gypsum slurry from which the board is formed; this is a particular problem at the edge margins of the board where the bottom mat is brought up and onto the upper surface of the board to define the edges of the uncut board. Inefficient exhaustion of air in this region can lead to voids in the edge-margins of the cut boards, reducing the edge strength of the boards.
The problem of voids in the edge margins has been dealt with by increasing the fiber diameter of the mat, particularly the bottom mat (to for example 16 μm), allowing easier exhaustion of air and penetration of gypsum slurry but reducing board strength. However, the use of higher diameter fibers has been found to decrease the strength of the mat. Reduction of the mat substance (weight/unit area), which would allow the gypsum slurry to penetrate the mat more readily, would lead to an unacceptable reduction in board strength.
The need to allow sufficient time for the gypsum slurry to penetrate the mat means that the line speed of the plasterboard manufacturing line is lower than would be the case were adequate exhaustion of air from the edge margins easier.
It has been desired to provide a GRG building board which can be manufactured at relatively high speed, is of high strength by virtue of using a mat of relatively low diameter fibers and the edge margins of which have a low level of voids.